1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection device and a method for producing the same. In particular, it relates to improvements of a discharge supporting electrode provided in an ESD protection device to promote electrostatic discharge.
2. Description of the Related Art
An overvoltage protection device which is of interest to the present invention is described in, for example, Japanese Unexamined Patent Application Publication No. 2008-85284 (Patent Document 1).
Patent Document 1 describes a material for an overvoltage protection device serving as a discharge supporting electrode for promoting discharge, the material containing a non-conductive powder (for example, silicon carbide with a particle diameter of 1 to 50 μm), a metal conductive powder (for example, copper with a particle diameter of 0.01 to 5 μm), and an adhesive (for example, a glass powder).
Patent Document 1 also describes a method for producing an overvoltage protection device, including a step of forming a material paste by evenly mixing a non-conductive powder, a metal conductive powder, and an adhesive at particular ratios, a step of applying the material paste onto a substrate by printing, and a step of subjecting the substrate to a firing treatment (temperature: 300° C. to 1200° C.)
However, according to the technology described in Patent Document 1 in which a glass powder is used as an adhesive, the glass component may become unevenly distributed depending on the dispersion state of the glass powder, possibly resulting in insufficient bonding between the metal conductive powder and the non-conductive powder. Accordingly, there is a problem in that shorting failure easily occurs especially in the event of high-voltage ESD.
Regarding this issue, International Publication No. 2011/040437 pamphlet (Patent Document 2) describes use of metal particles and semiconductor particles having a glass network-forming component on their surfaces, by which a glassy matter is generated and bonds between metal particles are formed. Compared to the technology described in Patent Document 1, the technology described in Patent Document 2 improves the dispersibility of the glass.
However, according to the technology described in Patent Document 2, generation of glass is insufficient and metal particles may not sufficiently bond to each other. If ESD occurs in such a case, shorting failure readily occurs.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-85284    Patent Document 2: International Publication No. 2011/040437 pamphlet